The aerospace industry is subject to stringent requirements for accurately and continuously monitoring and maintaining the identity of individual parts or components which are combined to form the large subassemblies which comprise an aircraft. Present identification techniques employ either an ink or painted number or other identifying mark which is applied directly to the surface of the part or a metal tag which is wired onto the part. These techniques are inadequate because markings applied directly to the part can be either obscured or destroyed by subsequent manufacturing operations and metal tags must be removed for certain operations.
Infrared, non-destructive testing has been used in the past to inspect parts for internal defects. One method of using infrared analysis to inspect parts is disclosed in U.S. Pat. No. 3,504,525. This method involves spraying the part with a vinyl base carrier of a carbon pigment to form a constant, high emissivity surface which is easily removed from the part after testing is complete. After application of the coating, the part is heated with a suitable source of radiant energy and the temperatures of successive spots on the test surface are determined by scanning the coated surface with a radiometer. Output from the radiometer is then transmitted to an oscilloscope or other display device where any flaws in the part are displayed as an infrared image. The method of U.S. Pat. No. 3,504,525 is particularly concerned with controlling the emissivity of the test surface at a standard level by applying a protective coating that has uniform radiating characteristics to the test surface. An essential characteristic of the coating applied to the test surface is that it can be easily removed when the test is completed.
U.S. Pat. No. 3,020,745 also discloses the use of an infrared detector to test metal objects for flaws. In this method the area to be inspected is heated by induced eddy currents which uniformly increase the temperature of the test surface in the absence of flaws. At a flaw, the induced current is concentrated at the edges or corners of the flaw and a hot spot develops which is detected by an infrared detector. This method requires that the test surface be covered with a thick homogeneous coating having a high-emissivity. U.S. Pat. Nos. 4,468,136; 4,429,225; 4,037,473; 3,864,958, 3,808,439; 3,451,254; 3,499,153; 3,433,052, and 3,378,658 also disclose the use of thermographic techniques and apparatus to inspect surfaces for flaws and to measure film thickness. Additionally, "Troubleshooting Products through Infrared Thermography", which appeared in the Nov. 10, 1983 issue of Machine Design discusses the use of infrared thermography as an inspection tool. U.S. Pat. Nos. 4,471,822; 4,365,307, 4,343,182; 4,027,473; 3,314,293 and 2,846,882 disclose the use of infrared techniques and apparatus to measure the temperature of objects.